Wpdb query for comment meta for current post

You can join to the comments table and use get_the_ID():

// set the meta_key to the appropriate custom field meta key
$meta_key = 'rating_total';
$ratingtotals = $wpdb->get_var( $wpdb->prepare( 
    "
        SELECT sum(cm.meta_value) 
        FROM $wpdb->commentmeta AS cm
        JOIN $wpdb->comments AS c ON (c.comment_ID = cm.comment_id)
        WHERE cm.meta_key = %s AND c.comment_post_ID = %d
    ", 
    $meta_key, get_the_ID()
) );

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